In electronic power supplies, it is common to require electrical isolation between the source of power and the load. In particular, electronic systems powered from the AC mains will generally be required to provide a safety galvanic isolation between the equipment being powered, with its associated ground connection, and the AC mains, both the hot and neutral wire of the incoming AC power. The power flow is isolated by use of a transformer, typically driven by high voltage power electronics that switch the transformer at high frequency. In order to provide a closed feedback loop between the high voltage power electronics and the isolated low voltage load, an optocoupler is commonly used.
Because of noise and cost requirements, the typical use of these optocouplers calls for the current transfer ratio of the optocoupler to be on the order of unity, such that a current on the order of a milliamp on the secondary side of the power supply creates a feedback control current on the order of a milliamp on the primary side. Use of lower currents leads to poor bandwidth in the phototransistor and the LED. Higher currents are undesirable as the efficiency of the system is compromised by dissipating additional power in the feedback.
Optocoupler Technology
Optocouplers, as are typically used in power supplies consist of two semiconductor die, an emitter and a receiver, in a single package that allows for an optical coupling between the two die but no electrical path between them. The receiver is typically a silicon phototransistor, and the transmitter is a light emitting diode (LED) based on a compound III-V semiconductor such as aluminum gallium arsenide (AlGaAs).
The receiver phototransistor is typically a bipolar transistor of conventional design which is optimized to be sensitive to incident light. This is accomplished by extending the area of the collector-base junction. This junction acts as a photodiode that will create a light-dependent “leakage” current from collector to base, thus providing a base current to drive the device on. This construction allows the photocurrent in the photodiode to be multiplied by the current gain (beta) of the bipolar transistor. This increases the sensitivity of the device compared to a photodiode by two to three orders of magnitude. A disadvantage of the structure is that the photodiode also acts as a junction capacitor from base to collector of the bipolar transistor, with this capacitance being in such a configuration as to act as a feedback “Miller capacitance”, slowing the response of the device significantly.
The phototransistor is co-packaged with an LED emitter such that there is an optical path between the two die. The package typically has either four or six leads, depending upon whether the phototransistor has its collector, base, and emitter connected to pins (for a 6 pin configuration) or if only the collector and emitter terminals are connected (requiring four pins with two each for LED emitter and phototransistor receiver). The phototransistor is connected to pins on one side of the package, the LED to pins the opposite. This allows for a significant space between the leads of the two components as required for high voltage isolation. Although it is possible to create packages that can connect more than three connections on each side of the package, there is a significant economy of scale involved if the standard high volume optocoupler package is used.
In production, the repeatability of the optocoupler is not easy to guarantee. The LED emissivity has a minor statistical spread. The optical coupling from the LED to the photodiode portion of the phototransistor can vary somewhat, primarily due to mechanical tolerances of the packaging. The sensitivity of the photodiode has a minor variation. But being a bipolar transistor, the current gain (beta) of the phototransistor can be difficult to maintain over even a 2:1 range. If a high current transfer ratio (CTR) is desired, then the basewidth of the transistor will be made small to increase the beta. As this basewidth is reduced, the statistical variations in beta increase. The beta of the phototransistor tends to dominate the statistical variations in the optocoupler CTR.
In high performance photonic circuits, such as are used for high speed optical communication of digital data, it is unusual to use a phototransistor. Instead, a photodiode is used. Although the photodiode produces significantly less current than a phototransistor of comparable size, the combination of a photodiode with a precision amplifier can be designed for higher performance than a photodiode driving the base of an otherwise “open base” bipolar transistor. Techniques are typically employed to make the circuitry very wideband by operating the photodiode at a constant voltage, such that none of its photocurrent is used to charge or discharge the diode capacitance. The precision amplifiers used, because they convert an input current to an output voltage, are generically known as transimpedance amplifiers, or TIAs. The use of sophisticated photonic circuits has never been viewed as cost-effective in power supply circuits, where the cheaper optocoupler is “good enough” and an order of magnitude cheaper than schemes used in optical digital communications.
Power Supply Overview
The offline power supply generally constitutes two separate circuits. The circuitry on the input high voltage supply (DC or AC), generally referred to as the primary side, there is one or more power switching element, typically bipolar transistors or MOSFETs, and circuitry to turn them on and off. The power transistor(s) drive the primary of a power transformer. In order to power the primary side circuitry, a low voltage supply rail is typically created, often with an additional winding on the transformer which may be rectified and filtered.
An isolated secondary winding of the transformer connects to the output circuitry, generally referred to as the secondary circuit. The secondary circuit includes rectifiers to convert the switching AC from the transformer to a DC current, and there will typically be either capacitors or an LC network to smooth the switched DC back to a low ripple constant voltage. Typically the power supply includes a feedback mechanism to precisely control the output voltage. The information from the secondary side output must be fed back to the primary side in order to close the feedback control loop.
The most common approach is to compare the output voltage to the desired output, and create an integrated error signal. This error signal is fed from secondary to primary via an optocoupler. A control circuit on the primary adjusts the switching pulses so as to minimize communicated error. Typically the control circuit is a pulse width modulator, as is well known in the art. This is generally termed primary side control, as only the reference and error amplifier reside on the secondary side. Alternative implementations may modulate the analog feedback informations, as for example an AM or FM signal that can be transmitted across the isolation barrier via a simple transformer.
A second approach to controlling the power supply, secondary side control, is to place the pulse modulating circuitry directly on the secondary side. This allows a very direct connection between the circuitry measuring the output and the analog control circuitry that controls pulse width. In this case, a simple digital (on/off) pulse signal needs to be sent back to the primary side to control the power switch. This digital information is generally sent back across the isolation boundary via a transformer. Communicating the pulse information via optocoupler is impractical because of the slow speed of the phototransistor. A disadvantage of the secondary control approach is that it is necessary to provide an additional pulse generating circuit on the primary for initial system start-up, as the system must “bootstrap” itself and begin switching and delivering power to the secondary circuitry until the secondary circuitry is sufficiently powered to begin sending pulses back to the primary. Whittle's “kick start” circuit (U.S. Pat. No. 4,695,936) is a classic solution, directly pulsing a primary power transistor only in the absence of secondary control pulses.
Independent of the choice of primary or secondary control, there is a need for the primary side circuitry to bootstrap itself from a high voltage DC supply. Because high voltage devices are cost-prohibitive on integrated circuit devices, a typical solution, referred to as undervoltage lockout (UVLO) is to place a high value bleed resistor (several megohms) from the high voltage rail to the positive supply of the primary side circuit. A reservoir capacitor is charged by the current in this bleed resistor while the control circuitry is held initially in a low current state. When the voltage across the reservoir capacitor approaches the maximum allowable voltage for the control circuitry, the circuit is powered on and will run briefly on the energy stored in the capacitor. Ideally, the power supply will start during this time and an alternate source of supply current can be provided from the operating power supply. Should the reservoir capacitor be significantly discharged, the circuit re-enters its low current “off” state and the reservoir capacitor is allowed to resume charging for another attempt at startup.
A prior art flyback power supply using an optocoupler is illustrated in FIG. 1. The primary side circuit 1 accepts a positive high voltage from terminals BULK to RTN. The secondary circuit 2 provides a positive output voltage between terminals OUT and GND. The two input terminals and the two output terminals are galvanically isolated, the two circuit block being coupled magnetically through the transformer T1 (with windings T1a, T1b, and T1c) and via optocoupler P1 (with LED emitter P1a in the secondary circuit 2 and phototransistor receiver P1b in the primary circuit 1).
In the primary circuit 1, capacitor C1 acts as a bulk storage capacitor. Resistor R1 is used to allow control circuit U1 to start with an undervoltage scheme, and is designed to provide current only sufficient to allow for initial startup. Capacitor C2 acts as the local reservoir capacitor for the undervoltage lockout scheme and as a local decoupling capacitor for controller U1. Controller U1 drives power MOSFET M1, which switches the bulk voltage across the transformer primary T1a. The switching action causes energy to be stored in the inductance of transformer T1, and upon switching MOSFET M1 off, that energy is transferred to secondary windings T1b and T1c, which creates positive voltages across the capacitors C3 and C2, respective.
At startup, controller U1 is held off at low current to allow capacitor C2 to be charged through resistor R1. As capacitor C2 reaches an appropriate voltage, typically 10V-20V, controller U1 turns on and begins switching MOSFET M1. This action allows transformer winding T1c and diode D2 to begin acting as a low voltage power source to provide controller U1 with power on an ongoing basis.
The voltage on the secondary output terminal OUT increases as transformer secondary T1b transfers energy into output capacitor C3. A voltage divider comprising resistors R2 and R3 is used to compare a fixed fraction of the output voltage at terminal OUT to the reference voltage of a reference/amplifier circuit 3, typically a circuit similar to the TL431. A compensation network 4 is used to stabilize the overall control loop. A resistor R5 from the output of reference/amplifier 3 to the LED emitter P1a of optocoupler P1 acts as a voltage-to-current conversion to drive the LED with a feedback signal.
The LED current in P1a is a feedback control signal that is transmitted optically to the phototransistor P1b on the primary. The collector current in the phototransistor P1b is dropped across a load resistor R7 to create feedback voltage that serves as an input into controller U1. Controller U1 is typically a PWM control integrated circuit, which will convert the feedback voltage signal either directly into a pulse-width signal or, more commonly, into a current command signal for a current-mode PWM. As shown in FIG. 1, such a current mode would be accomplished by turning on MOSFET M1 at the beginning of each cycle and monitoring the current flowing jointly in MOSFET M1 and transformer winding T1a by monitoring the voltage across a current sense resistor R6. When the voltage across current sense resistor R6 is comparable to the value commanded by the feedback current flowing in phototransistor P1b, then the drive voltage to MOSFET M1 is terminated and MOSFET M1 is shut off until the next cycle.
Further Integration
Some effort has been made to further integrate the power system by, for instance, placing a secondary side reference/amplifier die, typically a TL431 or equivalent, in the same package as the LED and phototransistor. Although this can reduce the total number of packaged components needed to implement the system, this has minimal impact on cost because the LED requires a large bandgap semiconductor die (e.g. aluminum gallium arsenide) whereas the reference/amplifier die that drives it will typically be silicon, so it is not practical to integrate the functions on a single die. Worley (U.S. Pat. No. 6,885,016) further attempted to integrate secondary circuits by using silicon as the light-emitting element from within a secondary side integrated circuit (theoretically possible but not practical within the confines of standard silicon integrated circuit technology), though this also forced the use of a time-integrating photodetector on primary side to overcome the limitations of the inefficient emitter, resulting in performance much more limited than the standard phototransistor.
Both the phototransistor and the power supply control circuits are typically implemented in silicon, but it is not generally accepted as feasible or practical to implement both functions on a common die. The phototransistor is typically a relatively large die (in order to absorb as much of the available photons as possible) and is therefore economically made in a very simple process in which the cost per unit of area of silicon is small, whereas the economics of the control circuit, typically a relatively complex integrated circuit, tends to follow the general trends of integrated circuit economics, where ever increasing cost per unit area of improved silicon processes is offset by ever shrinking size of the circuits. Further, placing an integrated circuit under a bright source of light can create circuit problems that are difficult to manage, as the same physical phenomenon that creates photocurrents in the phototransistor generally will also generate photocurrents elsewhere on the die.
Photocurrents produced by light intentionally shining on an integrated circuit are not generally predicted by the methodologies used to design and verify integrated circuits, and therefore are seen as unpredictable and as a source of considerable risk when integrated circuits are commingled with photonic components on a single die. Although techniques have been developed to allow for the creation of high speed digital communications via on-chip photonic circuits, these techniques are not implemented in mainstream design tools used for standard analog and mixed signal circuits for power supplies.
It is the objective of the present invention to provide a practical integration of the photonic and control circuits on the primary side of the power supply.
It is a further objective of the present invention to significantly lower the current requirement of the feedback circuitry on the primary side of the power supply.
It is yet a further objective to improve the performance and to lower the statistical variability of the feedback circuitry on the primary side of the power supply.